Tag
#instrumentation
14 topics across all disciplines.
Valve actuation and fail position — deciding what happens when everything stops
Pneumatic, electric and hydraulic actuators, why a spring is what makes fail-safe possible, how fail open and fail closed are chosen per service, and what a positioner actually does.
What a transmitter is, and why it sends 4–20 mA
How a field instrument turns a physical quantity into a number the control room can use, why the signal starts at 4 mA instead of zero, and how to pick a measurement principle.
Measuring temperature and pressure — thermowells, thermocouples and gauges
Why a thermocouple needs matching extension wire and cold junction compensation, why an RTD should never be wired with two wires, what a thermowell costs you in response time, and why a pressure gauge needs a siphon on steam and a snubber on a reciprocating pump.
Measuring flow — orifice plates and the alternatives
Why most flow measurement is really a pressure measurement, what the square root relationship means for turndown, why straight lengths are a measurement requirement rather than a layout preference, and how to choose between orifice, vortex, magnetic, Coriolis and ultrasonic meters.
Measuring level — why most level readings are really weight
How differential pressure, displacers and radar each infer level, why a wet leg that partially drains makes the tank read full, what a bridle is for and how it lies on hot service, and why a gauge glass disagreeing with the transmitter is information rather than a nuisance.
Reading a P&ID — the drawing every discipline works from
How a P&ID differs from a PFD, decoding instrument tags and bubbles, following a control loop across the sheet, and the details that decide what gets built.
Control loops and PID — what the three terms actually do
How a feedback loop works, what proportional, integral and derivative each respond to, why controller action is the error that breaks loops completely, and how to tell a tuning problem from a valve problem.
Control valves — fail position, cavitation and why oversizing ruins control
What a positioner actually does, why the fail-safe position is a process safety decision that must be verified rather than assumed, the difference between cavitation and flashing and what each does to the trim, and why a valve sized for the line rather than the duty controls badly.
Safety instrumented systems and SIL — the layer that acts on its own
Why protection is kept separate from control, what a SIL number actually measures, how voting trades spurious trips against missed ones, and why a bypass left in is the classic failure.
Intrinsic safety — protecting the circuit instead of the enclosure
Why instruments use a protection concept that limits energy rather than containing an explosion, how barrier and field device parameters are matched, why cable length is a safety limit rather than a signal one, and what a zener barrier depends on absolutely.
Instrument installation and loop checking — where most readings go wrong
Hook-up drawings, why impulse lines must slope the right way, what a manifold is for, and the commissioning sequence that catches problems before the plant does.
Calibration and accuracy — what a certificate actually proves
The difference between ranging a smart transmitter and calibrating it, why as-found readings matter more than as-left ones, what turndown does to accuracy, how hysteresis hides from a one-direction check, and why traceability is what turns a number into evidence.
Control system architecture — what is actually behind "the DCS"
How a signal gets from a transmitter to a screen, what marshalling is for, why DCS and PLC are different animals, and why an air gap is almost never what people think it is.
Analysers and sampling systems — where the real problem usually is
What an analyser measures that a transmitter cannot, why the sample system causes most of the failures, how transport lag limits what you can control, and why the take-off point decides everything.